The Institute of Electrical and Electronics Engineers (IEEE) inter alia sets the standards for communication devices interchanging information using the Ethernet protocol in order to enable for example different manufacturers to produce devices complying with the same specifications and thus being compatible to each other. For example 10 BT is a well known Ethernet standard protocol for transmitting digital information at a transmission speed of 10 Mbit/s.
The IEEE 802.3 standard defines the requirements for transmitting information using the 10 BT protocol on unshielded twisted pair (UTP) lines, wherein numerous details are specified in the sections of the standard. For example section 14.3 of IEEE 802.3 “MAU electrical specifications” specifies a differential output voltage between 2.2 Volts and 2.8 Volts on a load of 100Ω. Wherein for a load with varying impedance in the range of 86.8Ω to 115Ω it is specified that the transmitter has to provide an output impedance in a range of approximately 80Ω to 120 Ω.
As a widely spread supply voltage for integrated circuits (ICs) is 3.3 Volts active impedance line drivers are also implemented using operational amplifiers operated at this nominal voltage, wherein the supply voltage often is allowed to vary by 10%. Accordingly a nominal supply voltage of 3.3 Volts can drop to 3.0 Volts. Regarding the requirements of the IEEE 802.3 standard for 10 BT protocol such a voltage drop is considerable in view of maintaining the required output voltage swing and output impedance of the line driver. In particular the design of the output stage of a 10 BT line driver becomes difficult, because the output stage of the line driver controls the output voltage and output impedance of the driver. Accordingly the design of an active output impedance line driver for transmitting symbols using the 10 BT protocol gravitates around trading output swing for control of the output impedance, with the intention to keep the transistors in the amplifier's output stage out of linear operation, because a linear operation of the transistors can cause gain drop or even instability of the amplifier and thus of the entire circuit. That is, in order to maintain the transistors operate in a non-linear region, their source-drain voltage must be large enough. Hence when designing an output stage of a 10 BT line driver operated at a supply voltage of 3.3 Volts with an admissible variation of ±10% care must be taken to provide for a large enough source-drain voltage of the output stage transistors.
A conventional solution of a 10 BT line driver is shown in FIG. 1. The topology of circuit 100, i.e. the arrangement of elements, is symmetrical to dotted line 101 or the common mode voltage supply VCM. Accordingly for an element in the upper half of the drawing, i.e. any element above dotted line 101 having an even reference numeral there is a corresponding element having an uneven reference number (increased by 1). The operation of the two identical circuit portions is also very similar, wherein the circuit portion shown above dotted line 101 processes the positive signal portion output at terminal VOP and the circuit portion below dotted line 101 processes the negative signal portion of the output signal VOUT, and wherein the positive and negative portions of the output signal are symmetrical with reference to a common mode voltage.
Circuit 100 takes a voltage VIN as input signal representing the symbol to be transmitted and processed to be output as output voltage VOUT, which can be coupled to a transmission line represented as an ohmic resistor RCABLE.
The circuit comprises single ended operational amplifiers 110, 111 each coupled with its positive input terminal via resistor R3 120, 121 to a common mode voltage supply VCM. Input signal VIN is fed into the negative input terminals of amplifiers 110, 111 via resistors R1 130, 131. A negative feedback loop around each amplifier comprises resistor R2 and optional capacitor CFB 150, 151 and a positive feedback loop comprises resistors R4 160, 161 and optional capacitors 170, 171 and termination resistors RT 180, 181.
In case that circuit 100 is operated at a supply voltage of 3.3 Volts, which can drop to 3.0 Volts, and the output swing reaches close to its maximum, i.e. close to the supply voltage, then the value of termination resistor RT, which is used to control the output impedance of the line driver, decreases considerably making the positive feedback loop almost as strong as the negative feedback loop, which can lead to an instable operation of the circuit.
The risk of instable operation of the amplifiers is reduced by introducing the capacitors CFB 150, 151 into the feedback loops as shown, but which does not guarantee proper operation in all situations. Thus a supply voltage much higher than the required output swing of VOUT usually is used, for example a supply voltage of 5 Volts.
Thus there is a need for a 10 BT line driver operable at a voltage of 3.3 Volts, which also accurately operates in case the supply voltage drops to 3.0 Volts, and which accurately generates a 10 BT signal within the IEEE 802.3 specifications in these operating conditions.